The present invention relates generally to an air filter and, more particularly, to an adsorption air filter having a self-supporting shaped structure of adsorber particles and a method for the production of such an air filter.
U.S. Pat. No. 5,332,426 issued to Tang et al.(hereinafter "Tang") on Jul. 26, 1994 discloses a self-supporting filter structure made of adsorber and binder particles, having not only good filter performance and good adsorption capacity, but also a very low pressure drop, i.e. high air permeability. This feature is important for use as an air preparation filter, indoor air filter, automobile ventilation filter for the interior of an automobile, and as a filter in disposable gas masks.
In this connection, Tang proposes a self-supporting shaped structure of adsorber particles of irregular form, which are joined to one another by thermoplastic binder particles, also of irregular form, the binder particles being substantially smaller than the adsorber particles. In the interest of a low pressure drop, a plurality of adsorber and binder particles are welded together into agglomerates whose shape and distribution are also irregular. They possess an average size of at least 15 mesh. Located between these agglomerates are gaps which are free of material and constitute a pore volume in the filter of approximately 70 to 85%. The result is therefore a low-density open matrix.
The production method involves preheating the adsorbing particles to a temperature above the melting range of the binder; subsequently mixing the heated adsorber particles with the thermoplastic binder particles to form agglomerates with a fineness of at least 15 mesh; sieving out defined agglomerate fractions; shaping the agglomerates into the desired flat shaped elements in an open mold; and heating the flat shaped element to a temperature above the melting range of the binder. The agglomerated filter member is available after cooling.
Configuring the agglomerates requires two separate process steps, namely heating before and after mixing, Which requires a considerable outlay of time and energy; durations of 40 minutes are cited for each process step. To avoid density gradients inside the shaped structure, it is advised that the mold be inverted at least once as the mixture is being heated. This stipulation again makes the production process longer. With the known procedure, yet another process step is necessary in order to add an adsorption auxiliary, e.g. K.sub.2 CO.sub.3 as an aqueous solution, to the adsorber particles prior to the first heating. This treatment is used for chemisorption of acid gases by neutralizing them.
In the air filter itself, the relatively large spaces (pores) between the agglomerates contribute to a definite decrease in pressure drop. In practice--and as is also apparent from the Figures of Tang--gaps of such large dimensions mean that a considerable quantity of gas molecules intended for absorption pass through these gaps without coming into the physical influence region of the absorptive surfaces of the adsorbent. Therefore, this effect worsens the adsorption kinetics of the filter. The present invention is directed to providing an air filter and a method for its production that eliminates each of the aforesaid disadvantages.